Children commonly present with acute inversion injuries of the ankle. Radiographs occasionally show small fragments under the tip of the lateral malleolus. The purpose of our study was to clarify their origin. A prospective standardized clinical examination and a magnetic resonance imaging were carried out. Among the 116 included patients, seven of them showed an os subfibulare. They presented more pains on the lateral ankle area than the other children. We found also more injuries on the anterior talofibular ligament. The os subfibulare could be secondary to an accessory ossification but the lateral area of the ankle would be likely to be injured.